A resource and environmental problem such as the depletion of a fossil resource or a global warming problem is one of the serious problems in the 21st century. The establishment of an alternative resource technology that is environmentally friendly and abundant, and can persist has been required for solving such problem. Biomasses such as polysaccharides are each an organic resource that is present in the largest amount on the earth and renewable. Of those, cellulose is the most abundant biomass because the cellulose is annually produced in an amount of about 400 billion tons on the earth. Main applications of the cellulose include a fiber, paper, and a film. However, the applications of the cellulose are remarkably limited because of the following reason. The cellulose is extremely poor in meltability and solubility, and hence involves a problem in terms of forming processability. Accordingly, an additionally simple method of dissolving the cellulose has been required.
An N-methylmorpholine-N-oxide/water mixed solvent method (for example, Patent Literature 1), and a method for dissolution with a mixture of lithium chloride and N,N-dimethylacetamide (for example, Non Patent Literature 1) have each been known as a method of directly dissolving the cellulose without relying on any chemical change. The mixed solvent of N-methylmorpholine-N-oxide and water is the only solvent capable of directly dissolving the cellulose that has been used in the industry. The solvent needs to be heated to around 130° C. for dissolving the cellulose. The operation involves danger because the solvent may detonate at about 150° C. In addition, the dissolved cellulose is rapidly decomposed in such high-temperature region and hence an additive for preventing the decomposition is indispensable.
The mixture of lithium chloride and N,N′-dimethylacetamide requires, for example, the following pretreatment depending on the kind of the cellulose to be dissolved. A cellulose suspension in which the cellulose has been dispersed is heated at 100° C. or more for a long time period, or the cellulose is swollen by being immersed in water or an alcohol for a long time period in advance. Accordingly, it is difficult to dissolve the cellulose simply. In addition, the molecular chain of the cellulose may be cleaved in the heating step which leads to reduce its strength as compared with that of the cellulose before the dissolution. The mixture of lithium chloride and N,N′-dimethylacetamide has been limited to a use on a laboratory scale owing to those problems, and hence has not been put into industrial use yet.
A method of dissolving the cellulose involving using a high-concentration aqueous solution of sodium thiocyanate has been known as another method (for example, Patent Literature 2). The solvent system can dissolve a cellulose called as type II which has been treated with an aqueous solution of sodium hydroxide or an amorphous cellulose not in a crystalline state, however, the solvent system cannot be applied to a natural-type cellulose. In addition, a heating step at 100° C. or more is needed in the method as well and hence the strength of the resultant cellulose may reduce.
A method of dissolving the cellulose using an imidazolium-based ionic liquid has been proposed in recent years. The method has been attracting attention because of its high cellulose-dissolving power and small environmental load (for example, Patent Literature 3). However, a cellulose solution using the ionic liquid has a high viscosity and is liable to gel, and hence the resultant cellulose solution involves a problem in terms of forming processability. In addition, a method, which includes adding an amino base to an ionic liquid so that the liquid expresses cellulose solubility or the cellulose solubility of the liquid is improved, has been known (for example, Patent Literature 4). However, the solubility of the cellulose is limited in the method as well and hence needs to be additionally improved. In addition, the addition of the amino base may decompose the cellulose. A method, which includes adding an amine to a mixed liquid of a tetraalkylammonium salt and dimethyl sulfoxide (DMSO) to dissolve the cellulose, has been known (for example, Patent Literature 5). However, there is room for improvement on the solubility of the cellulose in the method as well. In addition, the cellulose may be decomposed by the amine in the method as well.